Exhaust emissions from combustion engines have increasingly become an environmental concern. As environmental laws become more restrictive each year, numerous methods and apparatuses have been developed to reduce the level of pollution materials in the exhaust.
Exhaust emissions are derived from the complete and incomplete combustion of fuel and air. These exhaust emissions have an unstable molecular mass as a result of the combustion process. Primarily, the exhaust emissions consist of a mixture of gases and low molecular weight carbonic particles. Sometimes the exhaust can include traces of oil lubricants. Depending on the fuel being burned by the engine, carbon and various formations thereof typically account for a majority of the particulate content in exhaust, as much as 80 percent in the case of diesel engines. Pollution control efforts, therefore, have focused on the reduction of particulate matter found in exhaust.
Diesel engines are credited with largely contributing to air pollution because they have a higher rate of exhaust emissions containing oxides of nitrogen (NOx) and carbonaceous particulate matter compared with that of gasoline engines. However, because of the durability and economy of diesel engines, they are frequently chosen over traditional spark-ignition gasoline engines in a significant number of trucks, buses, agricultural equipment, locomotives, ships, and stationary applications. Diesel engines also maintain a large popularity with European automobile manufacturers.
Apparatuses for reducing particulate matter in engine exhaust are known in the art. In a typical exhaust filter for a combustion engine, filtering of particulate matter is achieved by directing the exhaust through a porous material. The porous material captures particles that are large enough to become caught in the diameter of its pores. The remaining particles that are not captured within the porous material are discharged into the atmosphere.
Because of the inherent disadvantages in utilizing particle-capturing porous materials as filtering elements, the exhaust filtering systems in use today have several shortcomings. One such shortcoming is the compromise that must be made between filtering effectiveness and engine performance. For optimal filtering effectiveness, the porous material used in the system has a pore size that is small enough to capture the smallest particles discharged by the engine. In contrast, optimal engine performance demands large pores (or no filtering system at all), allowing for a free flow of exhaust through the exhaust line. Because of these competing interests inherent with engine performance and pollution, engine performance and filtering effectiveness have always been compromised.
Furthermore, in certain operating conditions, it is imperative to relieve the back pressure in the exhaust line that necessarily results from a restrictive porous filtering system. To that end, some filters are designed so that they provide an alternate unrestricted bypass for the flow of exhaust. This yields a more expensive product, and diminishes filtering effectiveness.
Yet another disadvantage of the particulate filters in use today is the variation in engine efficiency that necessarily results from extended use of the filtering systems. As particles are caught and build up in the filtering system pores, exhaust is increasingly restricted. Eventually, the filter will need to be replaced, or will need to undergo regeneration, whereby the filter is heated to a temperature that incinerates the particulate buildup, clearing the porous material of obstructive particles.